The present invention concerns a high strength, low thermal expansion alloy wire. More specifically, the invention concerns a high strength, low thermal expansion alloy wire having a tensile strength of 100 kgf/mm.sup.2 or higher and used as material for central section wire of low relaxation overhead power transmission line.
As the central section wire of the overhead power transmission line there has been used Fe-Ni based alloys or Fe-(Ni+Co) based alloys such as "Invar", Fe-36%Ni, "Kovar", Fe-29%Ni-17%Co and "Super Invar", Fe-36%(Ni+Co).
Fe and Ni are essential for controlling thermal expansion and used in combination in the most suitable proportion for realizing desired thermal expansion coefficient at the temperature ranges in which the alloys are used.
From the view to increase the strength, suitable amounts of various elements such as C, Si, Mn, Ti, Cr, Mo, W and Nb are added to form alloys which are practically used for the purpose of enhancing solid solution to heighten the matrix strength, or facilitating deposition of carbides/nitrides or intermetallic compounds.
Production of wire from these alloys is carried out generally by the following steps: blooming or forging alloy ingots or slabs made by casting or continuous casting--hot wire rolling--surface treatment (acid pickling or peeling)--wire drawing--softening annealing/aging--plating. Wire drawing and softening annealing may be repeated several times. Optionally, further wire drawing is carried out prior to the plating so as to increase strength by means of work hardening.
Strict requirements are claimed on the central section wire of the low relaxation power transmission line, such as (1) high strength (tensile strength 100 kgf/mm.sup.2 or higher); (2) low thermal expansion (linear expansion coefficient, .alpha., up to 5.times.10.sup.-6 /.degree. C. in the temperature range from room temperature to 300.degree. C.); and (3) high elongation (1.5% or higher). In addition to these properties it is desired that the wire has (4) high rupture twisting (16 times or more). "Rupture twisting" means the number of rotation until rupture when an alloy wire with a gage length 100 times of the wire diameter is twisted at a rate of about 60 rpm. This is usually applied to testing the wire material for power transmission line.
In the conventional alloy wire made by working an alloy of known composition in an ordinary method of working can meet the requirements of (1) to (3) mentioned above, but it is difficult to keep the number of rupture twisting high. It is experienced that the number of rupture twisting is a property of significant dispersion, and therefore, it is necessary for providing reliable power transmission line to increase the number of rupture twisting to a higher level.
We have made research with the intention to provide a high strength, low thermal expansion alloy wire having improved number of rupture twisting without damaging other properties of the wire, and discovered that it is effective to carry out the above noted process for wire production by, in addition to the specifically chosen alloy composition, limiting the quantity of intergranular precipitations at finishing of hot wire rolling, more specifically, by suppressing the quantity of intergranular precipitation up to 2% (areal percentage) and by making the crystal grains to a specific fine state, more specifically, in the range of 5-70 .mu.m. Even though satisfaction of one of these conditions may give a wire material of desired properties, if both of them are satisfied, then the product will have better properties.
The requirements of the intergranular precipitation and the crystal grain sizes may generally be realized by heat treatment for solid solution of the material after wire rolling (with efforts to keep the crystal sizes small). Needless to say, heat treatment requires time, labor and energy, which increase production costs, and therefore, it is desirable to eliminate the heat treatment step.